Decoders for quadruphonic sound utilizing wave matching logic

ABSTRACT

Apparatus for decoding four separate channels of information transduced from a medium having only two separate tracks and presenting it on four loudspeakers to give the listener the illusion of sound coming from a corresponding number of separate sources. The realism is enhanced by a decoding system which accepts the two outputs from the medium, which may be a disc record, one for each track, separates them into four independent channels each carrying predominantly the information contained in the originally recorded sound signals, and, utilizing a novel wave-matching technique, derives control signals for controlling the gains of amplifiers associated with the four looudspeakers. The use of the wave-matching technique improves the separation of the four independent channels, particularly the generally &#39;&#39;&#39;&#39;front&#39;&#39;&#39;&#39; from the generally &#39;&#39;&#39;&#39;back&#39;&#39;&#39;&#39; signals, thereby to enhance the realism of four-channel simulation.

United States Patent [1 1 [111 3,784,744

Bauer Jan. 8, 1974 DECODERS FOR QUADRUPHONIC SOUND Primary Examiner-William C. Cooper UTILIZING WAVE MATCHING LOGIC [75] Inventor: Benjamin B. Bauer, Stamford,

Conn.

[73] Assignee: Columbia Broadcasting System, Inc.,

New York, N.Y.

[22] Filed: Feb. 24, 1971 [21] Appl. No.: 118,271

[52] US. Cl. 179/1 GQ, 179/100.4 ST, 179/100.1 TD [51] Int. Cl H04r 5/00 [58] Field of Search 179/1 G, 1 GP, 15 BT, 179/1 GA, 100.1 TD, 100.4 ST; 330/124, 127,129, 131,134, 135,138, 140,141

[56] References Cited UNITED STATES PATENTS 3.632.886 l/l972 Scheiber 179/15 BT 3.708.631 l/l973 Grauereaux 179/1 GQ OTHER PUBLICATIONS 4 Channels and Compatibility by Schiebes Audio Engineering Society Preprint Oct. 12-15, 1970.

Assistant Examiner-Thomas DAmico Attorney-Spencer E. Olson 5 7 ABSTRACT Apparatus for decoding four separate channels of information transduced from a medium having only two separate tracks and presenting it on four loudspeakers to give the listener the illusion of sound coming from a corresponding number of separate sources. The realism is enhanced by a decoding system which accepts the two outputs from the medium, which may be a disc record, one for each track, separates them into four independent channels each carrying predominantly the information contained in the originally recorded sound signals, and, utilizing a novel wavematching technique, derives control signals for controlling the gains of amplifiers associated with the four looudspeakers. The use of the wave-matching technique improves the separation of the four independent channels, particularly the generally front from the generally back signals, thereby to enhance the realism of four-channel simulation.

20 Claims, 8 Drawing Figures PATENTED 81974 SHEET 1 OF 5 $38 m M Q Q k NE KEV Q k 9w NE BE ENE mm EN JEN I M Q an $58 i M H wi EQR 3 RH Y m 4 E N w M E v 5 3 m m B. N w M M E B Y B M M m QNQNMN E 3 H? mm INVENTOR.

A T TOR/V5 Y BE/VJA M/N B. BAUER PATENTEDJAH 81974 PATENTEU 314 SHEET 4 0f 5 INVENTOR. BENJAM/N B. 8A um ATTORNEY DECODERS FOR QUADRUPHONIC SOUND UTILIZING WAVE MATCHING LOGIC CROSS-REFERENCE TO OTHER APPLICATIONS This invention is related to the subject matter of the following applications, all of which are assigned to the assignee of the present invention: Ser. No. 40,510 filed May 26, 1970, now abandoned in favor of Ser. No. 164,675 filed July 21, 1971; Ser. No. 44,196, filed June 8, 1970, now US. Pat. No. 3708631; Ser. No. 44,224 filed June 8, 1970, now abandoned in favor of Ser. No. 251,544 filed Apr. 21, 1972, now abandoned in favor of Ser' No. 328,814 filed Mar. 10, 1973; Ser. No. 44,224 also now abandoned in favor of Ser. No. 185,204 filed Sept. 30, 1971; Ser. No. 44,224 also now abandoned in favor of Ser. No. 185,050 filed Sept. 30, 1971; Ser. No. 81,858, filed Oct. 19, 1970, now abandoned in favor of Ser. No. 251,636 filed May 8, 1972; and Ser. No. 112,168 filed Feb. 3, 1971 now US. Pat. No. 3,745,252.

BACKGROUND OF THE INVENTION This invention relates to apparatus for recording and reproducing four separate channels of information on a medium having only two independent tracks, and more particularly to apparatus for reproducing such information and presenting it on four loudspeakers to give the listener the illusion of sound coming from a corresponding number of separate sources. More particularly, the present invention is concerned with an improved decoder for four-channel sound, also known as quadruphonic sound, recorded on a two-track medium in accordance with the method described in the aforementioned co-pending application Ser. No. 44,224, and similar systems.

The recording method disclosed in application Ser. No. 44,224 is based on an encoding function which results from passing four signals associated with the four channels of sound (which, for convenience, are identified as L L,,, R and R for left-front, left-back, right-front and right-back, respectively) through six all-pass phase-shifting networks and thereafter combining them in appropriate proportions to produce two composite signals, L and R These composite signals may be transmitted over two lines, or recorded and reproduced over a two-channel recording medium, such as a stereophonic disc record, and subsequently transduced and presented for decoding into four simulated channels of sound by suitable decoding apparatus, one form of which is described in the aforesaid application Ser. No. 44,224. Without here going into a detailed description of the encoder, suffice it to say that the phasors corresponding to the composite signals L and R bear the relationship portrayed in FIG. 1 as phasor groups and 12. It will be noted that the signals L and R contain predominant signals L, and R respectively, with each containing a lesser proportion of L and R signals at 90 out of phase relative to each other, with L,, leading the L, signal in L, and R leading the R, signal in R It will be recognized that the signals L,, L R, and R,, are usually incoherent since they proceed from different instrumental or vocal groups and, accordingly, the phasor diagrams l0 and 12 (and others to which reference will be made as the description proceeds) represent the phase relationships of the common in-phase frequency components of the original signals.

It should also be noted that usually the channels L; and R; are associated with left and right loudspeakers respectively arranged at the two front comers of a room or listening area, while the signals L and R are associated with respective loudspeakers positioned at the left and right back comers of the listening area.

It is shown in application Ser. No. 44,224 that to accomplish decoding of the signals L and R depicted in FIG. 1, which typically will have been recovered from a tape or disc record by conventional playback methods and applied to the decoder directly, or after broadcasting and reception through a two-channel broadcasting system, they are first passed through respective all-pass phase-shifting networks (herein referred to as ill-networks) which introduce phase angle shifts, as a function of frequency, without substantial alteration of the amplitude of signals, identified by the expressions (ll! and (ll! 0), respectively, the expression in parenthesis identifying the angle vs. frequency function of each network. The reference angle III is arbitrarily chosen, the only requirement being that this reference angle is substantially the same in all phase-shift networks embodied in the decoder. Although a convention that the phase-shift angles are lagging has been observed, leading phase-shifts could also be employed, as long as the same convention is observed throughout the decoder. This phase-shift operation (which is also utilized in the present decoder) causes the phasors ap pearing at the output terminals A and B of the ill-networks to appear in the positions depicted by the diagrams 14 and 16. It will be observed that the magnitude and relative internal positions of the various frequency components have been retained (although in reality they are displaced by the reference angle ll! which is a function of frequency, from the phasor groups 10 and 12). To signify this relative phase displacement, the phasors appearing in groups 14 and 16 are distinguished by primes and, in the description to follow, each time a signal undergoes a phase-shift in a Ill-n6- towrk, another prime is added to its symbol. Thus, the phasor group 14 contains L, as a predominant signal, with the signals .707L' and .707R', appearing in the same relative relationship with respect to L f, and phasor group 16 contains R, as a predominant signal along with the signals .707R and .707L' also in the same relative phase relationship as found in the phasor groups 10 and 12. By virtue of the 90 differential phase-shift, the phasor groups 14 and 16 are positioned in phase with respect to each other in a manner to be linearly added and subtracted so as to derive two additional signals at terminals C and D, represented by phasor groups 18 and 20, respectively. The signal 18 contains predominantly the L,, signal, and signal 20 contains predominantly the signal R',,, with both contain ing signals .707L'; and .707R' in the relaive phase relationships illustrated by their respective phasors. It is evident, then, that by the relatively simple process of decoding disclosed in the aforementioned co-pending application (which is also used in one embodiment of the decoder to be described herein), there are produced four signals, 14, 16, 18 and 20, which respectively predominantly contain the original signals L;, R;, L and R These decoded signals are not pure or discrete original signals, however, each being diluted or contaiminated by two other signals. Nevertheless, when all four channels of the original program contain musical signals in concert, and the four decoded signals are reproduced over respective loudspeakers placed in the corners of the room or listening area, then as far as the listener is concerned there is sufficient mixing of the sounds in the room that the resulting overall sound effect is quite similar to the sound of the original four discrete channels, and a credible simulation of the original four-channel program results.

There are situations, however, in which it is desirable to provide an illusion of greater independence or purity of the decoded signals; for example, when theoriginal sound is present in one or two channels only, it is desirable to automatically attenuate the gain in those channels which originally were inactive, thereby to enhance the separation of the channels which are present. Two different logic and control systems for achieving this type of audible spatial enhancement are described in co-pending applications Ser. Nos. 44,224 and 81,858. It is a primary object of the present invention to obtain greater quadruphonic realism than that attainable with previously described methods while, at the same time, simplifying the circuitry for accomplishing it.

SUMMARY OF THE INVENTION Briefly, in accordance with one aspect of the invention, a simplification and reduction in cost of the decoder are achieved by providing at the input to the decoder two pairs of phase-shifting networks (instead of the two used in the decoders described in the afore-' mentioned applications), the networks of each pair being operative to introduce a relative shift of 90 between signals applied thereto, and applying the two composite input signals in parallel to the respective pairs, so as to produce four output signals bearing such a phase relationship to each other as to be combined by appropriate addition and subtraction to produce four signals predominantly containing left-front, left-back, rightfront and right-back information and having a phase relationship to each other favorable for application, without further phase-shifting, to their respective loudspeaker systems. Thus, effective decoding is achieved using only four phase-shifting networks, whereas in the decoders using two phase-shifting networks at the input required four additional phaseshifting networks, or a total of six, to achieve the same favorable phase relationship between the signals applied to the four loudspeakers.

Another aspect of the invention is concerned with improved logic circuitry for controlling the gains of the amplifiers associated with the four separate loudspeakers to enhance the realism of four-channel simulation. In accordance with this feature of the invention, four signals produced in the decoder and resembling the signals ultimately applied to the loudspeakers, are separately rectified and the resulting voltage waveforms compared and shaped to produce a pair of gain control signals which are utilized to control the gains of gain control amplifiers in the respective loudspeaker circuits. One of the signals controls in unison the gains of the amplifiers associated with the two front loudspeakers, and the other controls the two back loudspeakers in unison, the relative phases of the signals representing the independent channels, together with the nature of the control signals derived by wave-matching, improves the separation between the front and back sounds to enhance the realism of four-channel simulation.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantages of the invention, and a better understanding of its construction and operation, will be had from the following detailed description, taken in conjunction with the accompanying drawings, in which:

FIGS. 1A and 1B taken together is a schematic diagram of one embodiment of decoding apparatus embodying the invention, to which brief reference has already been made in discussing the background of the invention;

FIG. 1C is a series of phasor diagrams useful in explaining the operation of the invention;

FIG. 2 illustrates the control characteristic of a portion of the circuit of FIG. 1;

FIG. 3A and 3B taken together are curves useful in explaining the operation of the circuit of FIG. 1; and

FIGS. 4A and 4B taken together is a schematic diagram of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS tions are incorporated herein by reference to the extent necessary for understanding the operation of certain aspects of the herein described decoder; however, the description to follow is believed to be sufficiently complete to enable one skilled in the art to understand its operation Without recourse to the co-pending applications.

Reverting to FIG. 1, the present decoder is similar to those described in the aforementioned applications in that the composite signals L and R are applied to corresponding input terminals 22 and 24, subjected to a relative phase-shift of by a pair of ill-networks 26 and 28 to produce at terminals A and B the signals represented by phasor diagrams 14 and 16, respectively, and adding and subtracting the signals 14 and 16 in a pair of summing networks 30 and 32, each of which may be a simple resistor matrix, to derive signals at terminals C and D depicted by the phasor diagrams 18 and 20. The present circuit differs, however, from those described in the referenced applications in that it includes an improved logic and control circuit which results in greater enhancement of quadruphonic realism.

In accordance with the invention, the instantaneous amplitudes of the signals delivered to the four loudspeakers (in a manner to be described) are controlled by logic circuitry contained within the dotted line enclosure 34 in which a manner that a listener is given the illusion of four separate sources of sound (provided, of course, that four sources are present), and providing relatively good channel separation when less than four original signals are present in the composite L and R input signals. This is accomplished by applying the signals appearing at terminals A, B, C and D, depicted by phasor groups 14, 16, 18 and 20, respectively, to respective rectifiers 36, 38, 40 and 42 (which are preferably full-wave rectifiers) and smoothing the rectified signals with leaky integrators comprising capacitors 44, 46, 48 and 50 and resistors 52, 54, 56 and 58 respectively in parallel therewith. The decay-time characteristic of the four rectifier integrator circuits are substantially the same, the rise time being relatively rapid, of the order of 0.001 second, while the decay-time should be relatively longer, of the order of 20 milliseconds, although these times may be varied considerably.

The voltage produced by rectifier 33 is subtracted from the voltage produced by rectifier 36 in a subtracting circuit 60, which may simply be a summing resistor or junction, and the voltage produced by rectifier 42 is similarly subtracted from the voltage delivered by rectifier 40 in a similar subtracting circuit 62. The resulting signals from subtracting circuits 60 and 62 are again rectified by respective rectifiers 64 and 66, which are also preferably full-wave rectifiers, and integrated by respective R-C circuits 68 and 70. The rise time of these circuits, which are substantially equal, is of the order of milliseconds, and a decay time of the order of 400 milliseconds has been found satisfactory, although it has been determined by experiment that these values may be varied over an appreciable range without significant loss of performance.

The voltages appearing at junctions 72 and 74 may then be shaped in a pair of wave-shaping networks 76 and 78, respectively, each of which has a logarithmic transfer characteristic, and then applied in mutually reversed arrangement to a pair of subtracting circuits 80 and 82. That is, the output of network 78 is applied to the subtractive terminal of subtracting circuit 80 and to the adding terminal of circuit 82, and the output of network 76 is applied to the negative terminal of circuit 82 and to the positive terminal of subtractor 80. It may be noted at this juncture that in some cases the waveshaping networks 76 and 78 may not be necessary, in which case the voltages appearing at junctions 72 and 74 would be applied directly to the subtracting circuits 80 and 82 in the manner illustrated. Or, alternatively, the shaping networks 76 and 78 may be placed in circuit between subtracting circuit 60 and 62 and their associated rectifiers 64 and 66, the objective being to maintanin the relative amplitudes of the control signals as a function of position and relative magnitudes of the signals L L R; and R regardless of their total amplitudes. in either arrangement, the subtractor circuits 80 and 82 respectively deliver control voltages E and E; (having characteristics to be described hereinafter), the E, signal being used to control the gain of the loudspeaker circuits which handle the front loudspeaker signals, and the voltage B, being used to control the gain of the back loudspeaker circuits.

While the signals appearing at terminals A, B, C, and D, which, it will be remembered, correspond to the original quadruphonic signals L L,,, R,, and R respectively, may be applied to their respective loudspeaker circuits and still achieve satisfactory operation of the decoding and control circuit 34, it is preferable that they be applied through four additional ti -networks 84, 86, 88 and 90 which provide additional differential phase shifts so that the corresponding signal phasors 92, 94, 96 and 98 are in a relative phase relationship with each other more favorable to maintenance of the integrity of image formation at the adjacent loudspeaker pairs. It is to be understood, however, that reasonably satisfactory, but not ideal, performance is obtainable without ill-networks 84 90. As an alternative, the input signals to the left-front and right-front loudspeakers may be taken from terminals A and 8- (at the inputs of ill-networks 26 and 28) to help maintain the integrity of the phasors at the front loudspeakers when the ill-networks 84 are not used.

The signals delivered by ill-networks 84, 86, 88 and 90 are respectively applied to the input terminal of gain control amplifiers 1.0-0, 102, 104 and 106, the outputs of which are applied to loudspeakers 08, l 10, ll 12 and 114, respectively. These loudspeakers, which carry phase-shifted replicas of the signals L,, L,,, R, and R respectively, (which are designated L",, L",, R", and

R";, respectively) are placed in the corresponding corners of a room or listening area 116 as shown. Any other arrangement serving the taste of a particular listener may, of course, be used.

Each of the gain control amplifiers 1106 has a control electrode 118, 120, 122 and 124, respectively, application of a control voltage to which varies the gain of the amplifier. The gain control amplifiers may embody a variable gain vacuum tube, a suitable variable gain transistor, or any suitable vario-losser circuit, or other well known technique for providing variable gain in response to an applied voltage. For reasons to be explained, the control voltage E, from subtractor circuit 82 is applied in parallel to control terminals 118 and 1124 of amplifiers 100 and 106, respectively, to control the gains of the front loudspeakers, and the control signal E is applied in parallel to the control electrodes 120 and 1122 to control the amplification of signals applied to respective back loudspeakers. A suitable relationship of the gains of amplifiers 100 106 as a function of the voltage applied to their respective gain control terminals is depicted by the curve 128 in FIG. 2. It is seen in this exemplary case that when the gain control voltage is zero, the amplification factor is approximately 80 percent of maximum. When the applied control voltage E or E as the case may be, is strongly positive, the amplification factor approaches the 100 percent value, and if the control voltage is negative, the amplification factor decreases until at some value, -E the amplifier is cut off and removes the signal from its respective loudspeaker. The circuit is so arranged that when there is no input signal, or when all signals are present at equal strength simultaneously, the control voltages E and E; are at or near zero, causing all of the loudspeaker circuits to be at about 80 percent gain. This characteristic may be varied within reasonable limits, however, and it should also be understood that the shape of curve 128 is also subject to some variation without departing from the spirit of the invention.

The gain control amplifiers 100 106 have time constant circuits incorporated therein having a characteristic to allow the rapid application of an increasing control voltage in order that the amplification factor can be rapidly increased, but which allows the control voltage to decrease relatively slowly (as by slow discharge of a capacitor) so as to prevent the amplification factor from decreasing or being released" too rapidly. In a system which has been satisfactorily operated, a rise time of the order of 0.02 second and a release time of the order of 0.80 second have been found suitable. It is to be understood, however, that these values may vary over wide limits without significantly impairing the operation of the system.

The operating principle of the decoder and logic circuitry illustrated in FIG. 1 is based upon the phasor relationship of thesignals depicted by the phasor diagrams M, 16, i8 and 20, which are reproduced to a larger scale in FIG. 1C better to visualize the action of the logic circuit. If, for example, a left front signal L, only is present at terminal A, no signal appears at terminal B, as seen from inspection of right-front phasor group 16. It is seen, however, that the signal L, does appear in the two phasor groups 18 and 20, in the form of two identical phasors .707L,. If, on the other hand, a right-front signal R, is present in the phasor group 16, then this signal is not present in phasor group 14, but appears in equal magnitudes in phasor groups 18 and 20, but in an out-of-phase relationship. If both L, and R, are present simultaneously, then, since they are different sounds, they will be uncorrelated at terminals A and B, but will appear in either a positive or a negative correlation at terminals C and D. On the other hand, by similar reasoning, it may be shown that if only either one or both of signals L, and R, are present, they will be uncorrelated at the terminals C and D, but will be in either direct or inverse correlation at terminals A and B. An important aspect of the present invention in the utilization of a novel principle for recognizing these correlated or uncorrelated conditions without the use of conventional multiplying and integrating circuits. Rather, recognition of the correlated or uncorrelated condition is accomplished by instantaneously matching the waveforms of signals as they appear in the decoder.

Referring to FIG. 1C, let it be assumed that only the left-front signal L, exists, which after passage through the Ill-DCIWOl'kS 26, appears as phasor L,, shown in solid, heavy line. After rectification by the rectifier 36, this signal will produce a voltage for application to the positive terminal of subtracting circuit 60. Since no signal is present at terminal B, there will be no voltage developed by rectifier 38 and, accordingly, after subtraction in the circuit 60, a current due solely to the signal L, will appear at its output which, in turn, will result in a voltage being generated at junction 72 of integrating circuit 68.

Turning attention now to the phasor groups 18 and 20, it is seen that the two signals .707L,, shown in heavy solid lines, are identical in both phase and intensity. Therefore, the voltages at the output terminals of full-wave rectifiers 40 and 42 contributed by these two signals will be identical in magnitude and waveform with the consequence that when subtracted, one from the other, in circuit 62, zero current, and therefore zero voltage at terminal 74, results. It will be evident that after the signals appearing at terminal 72 and 74 are shaped by their respective shaping networks 76 and 78 and thereafter subtracted in the subtracting circuits 80 and 82, a negative voltage E appears at the output of subtractor 80. This voltage is applied in parallel to the gain control terminals 120 and 122 of amplifiers 102 and 104, respectively, causing these amplifiers, which actuate the back loudspeakers 110 and 112 to be partially or completely turned off, while at the same time amplifiers 100 and 106 feeding the front loudspeakers 108 and 114 will attain full gain condition. Therefore, the signal L, will appear to emanate principally from loudspeaker 108.

By similar reasoning, it is seen that if the signal R, only is present, identified by a heavy broken line in phasor group 16, only two other phasors will appear in the system, namely the phasor .707R, at terminal C, and a corresponding .707R',signal at terminal D, in the opposite direction. In this case, then, the output of rectifier 36 is zero, while the output of rectifier 38 is maximum, corresponding to the phasor R,. Subtraction of these two signals in subtractor causes a current to flow toward the subtractor, but since rectifier 64 is a full-wave rectifier, the voltage developed at junction 72 will be a positive voltage, as before.

Turning again to phasor groups 18 and 20, it is seen that the phasors .707R, at terminals C and D are oppositely directed; however, because rectifiers 40 and 42 are full-wave rectifiers, the voltages generated at their respective outputs will have the same polarity and will be very nearly identical. This is demonstrated in FIG. 3 which portrays, by way of example, an impulse contained in the signal R, applied to the system and appearing as two signals of opposite phase at terminals C and D. The positively directed signal at terminal C is shown in FIG. 3A as a decaying sine wave 130. Upon rectification, the portions of the wave below the time axis are reversed and appear above the time axis as shown by the corresponding dotted-line curves. After being smoothed by the integrating circuit, consisting of capacitor 48 and resistor 56, the resulting voltage is of the form shown in heavy dotted lines and identified as e Turning now to FIG. 3B, the initial impulse at terminal D is of the same magnitude as the one shown in FIG. 3A, but is reversed in phase. Again, after full-wave rectification, and smoothing by capacitor 50 and resistor 58, the voltage appearing at the output of rectifier 42 has the same magnitude and sense as the voltage appearing at the output of rectifier 40. Subtraction of one from the other in subtractor 62 produces a zero output, and consequently zero voltage at junction 74, as in the previous case. Thus, with either a L', or R, signal acting alone, there is a positive voltage at junction 72 and zero voltage at junction 74, resulting in turning on of the front loudspeakers 108 and 114 and turning off of the back loudspeakers 110 and 112.

When both L, and R, signals are present, remembering that these two signals are incoherent (even if they may be in concert), their instantaneous peak values as a function of time do not occur simultaneously; thus, after rectification by rectifiers 36 and 38 and subtraction at subtractor 60 there will be a net current through rectifier 64, resulting in a net voltage at junction 72. On the other hand, voltages developed by rectifiers 40 and 42 will continue to exhibit the same identity as described above with the result that the net output voltage at junction 74 is zero. In other words, even if there are two separate and distinct signals L, and R, supplied to the circuit in concert, only the front loudspeakers will be activated with increased gain and the back loudspeakers will be attenuated.

A converse action takes place when either one or both of signals L, or R, are applied to the decoder. In this case, there is a net control voltage at junction 74, and a zero voltage at the junction 72, resulting in a negative control signal E, and a positive control signal 13,, whereby the back loudspeakers 110 and l 12 are turned on, and the front loudspeakers 108 and 114 are turned off. It follows that as the various signals appear and disappear the appropriate gain control amplifiers are turned on and off in in response thereto.

Referring now to FIG. 4, there is shown in schematic form an alternative decoder matrix in which a simplification of the circuitry is achieved without sacrifice of performance and a modified form of the just-described logic circuit. It will be remembered that in order for the circuit of FIG. 1 to perform the decoding function it is necessary to first introduce a relative phase shift of 90 with l -networks 26 and 28 to place the signals L; and R, in quadrature. This relationship, per se, is not undesirable, except when these two phasors contain a common center signal. In order to form a virtual image of this center signal, in-between the two front loudspeakers, it is desirable that the two front signals remain in phase. This relationship may be obtained by various stratagems, the one used in the system of FIG. 1 being the incorporation of the four additional ill-networks 84, 86, 88 and 90. As has been suggested hereinabove, in a decoder which is not intended for audition of the highest quality it is permissible to dispense with ill-networks 84 90, instead connecting the two front loudspeakers (i.e., 108 and 114) to terminals A and B in stead of to terminals A and B, so that the two front loudspeakers receive the full, unmodified signals L and R respectively, in which the signals L, and R, predominate in proper phase relationship. However, even if this is done, there is a ill-function phase angle between the two front loudspeakers and the two rear loudspeakers, resulting in a certain degree of side and back image blurring and dissymetry. This latter effect is not sufficient to appreciably diminish the quality of the quadruphonic image and is the type of compromise which may be accepted in lower cost equipment. In the best professional equipment, however, it is important that the phasor groups are presented in a most favorable phase relationship.

This goal is accomplished, in the embodiment of the decoder shown in FIG. 4A. This decoder achieves the same relative phase relationship of the phasors applied to the loudspeakers obtained in the embodiment of FIG. 1A, but with a saving of two ill-networks; that is, the system of FIG. 4A, requires only four, instead of six, ill-networks.

In this decoder, two composite quadruphonic signals L and R recovered from a two-channel medium, and respectively protrayed by phasor groups and 12, are applied to the input terminals 150 and 152. These signals are in all respects identical to the corresponding signals applied to the input terminals of the decoder of FIG. 1A. Unlike the system of FIG. 1A, however, in which two ill-networks are used to properly position the phases of the two signals for subsequent manipulation, the decoder of FIG. 4A utilizes four such networks, 154, 156 158 and 160, two of which provide a phase shift of (1l1+0), and the other two introducing a phase shift of (1I1+9O), the L signal being applied in parallel to Ill-I16IWOIkS 154 and 156, and the R signal being applied in parallel to ill-networks 158 and 160. By reason of the relative 90 phase shift, the phasor groups 162 and 164 appearing at the outputs of til-networks 154 and 156, corresponding to the L signal, are in quadrature relationship, and similarly, the phasor groups 166 and 168 appearing at the outputs of iii-networks 158 and 160, respectively, corresponding to the R signal, are also in phase quadrature. Because the phase angle ll! generally varies with frequency, the phasor groups 162, 164, 166 and 168 do not bear a fixed angular relationship with respect to phasor groups 10 and 12, but inasmuch as the reference angle 11/ is the same for all of the networks, it is permissible to treat them as bearing a fixed relationship with respect to each other. It will be remembered that the signals L,, L,,, R, and R, are usually complex program signals and, therefore, the

phasor relationships within each phasor group represents the relationships of the same frequency components of the signals.

As in the circuit of FIG. 1A, the object of the decoder is to derive from the incoming signals L and R four separate signals predominantly containing the signals L',, L';,, R and R',, respectively, and to reproduce them over respective loudspeakers 170, 172, 174 and 176. To this end, the signals represented by phasor groups 162 and 168 are applied without change to respective gain control amplifiers 178 and 180, the outputs of which are applied to loudspeakers and 176, respectively. A signal predominantly containing L, is derived by adding .707 of the output of ill-network 156 and .707 of the output of lll-ntWOfk 160 in a summing circuit 182, the output of which may be represented by the phasor group 184, in which the signal L, predominates, with the signals L, and R, also being present but at a 3db lower level. This signal is amplified by gain control amplifier 186 and applied to loudspeaker 172. A signal predominantly containing the R signal is obtained by adding in summing circuit 188 .707 of each of the outputs of ill-networks 154 and 158, the resultant signal, represented by phasor group 190, being applied to a respective control amplifier 192 for application to loudspeaker 174. Thus, the loudspeakers 170, 172, 174 and 176 carry signals which have predominant information from the left-front, left-back, right-back and right-front channels, respectively. As with the decoder of FIG. 1A, each of these signals is contaminated with information from two other signals, but the contaminating signals being part of the same original program, their contribution is not unpleasant and, indeed, often provides an improvement in ambience or spaciousness of the musical selection.

It will be noted that the phasor groups 162, 184, 190 and 168 exhibit relative phase-shift relationships identical to the corresponding phasors of FIG. 1A, this desired relationship having been obtained with only four ill-networks, whereas the system of FIG. 1A requires six tit-networks to achieve the same result. Thus, the decoder of FIG. 4A offers the advantage of greater simplicity, with accompanying reduced cost. The arrangement comprising the four ill-networks and two summing circuits constitutes a satisfactory decoder for connection through suitable amplifiers and loudspeakers to produce a highly realistic and satisfactory rendition of the original quadruphonic program.

As with the decoder of FIG. 1A, in the interest of achieving the illusion of greater independence or purity of the decoded signals, it may be desirable to provide a logic and control circuit to provide enhance ment of the individual predominant signals. The logic and control circuit 34 of FIG. 1B may be used for this purpose, or the alternative control circuit within the dotted-line enclosure 200 in FIG. 4B may be used. The two signals at the outputs of I,lI-I'ltW0lkS 160 and 156, represented by phasor diagrams 168 and 164, respectively, are applied to a pair of gain control amplifiers 202 and 204, the gains of which are made to closely track each other by application of a common control signal to their respective gain control terminals 206 and 208. Although different in magnitude, thp outputs of these amplifiers, represented by phasors 168 and 164, have the same phase position as the similarly numbered phasors at the outputs of III-networks 160 and 156. These phasor groups exhibit predominant left and right front channel signals L; and R';, respectively, each also containing .707 of the signals L, and R',,.

By adding .707 of the output of amplifier 202 and .707 of the output of amplifier 204 in a summing circuit 210 a new signal is obtained which predominantly contains the left-back signal L' Similarly, by algebraically adding .707 of the output of amplifier 204 and -.707 of the output of amplifier 202 in summing network 212, another signal is obtained which contains predominantly the signal R',,. It will be observed that the latter two signals have the same phase relationship as phasors 184 and 190 in the decoder and, accordingly, have been similarly identified as 184 and 190'. It will also be noted that phasors 168', 190' and 164' have the same relative magnitudes and phase positions as phasors 16, 18, and 14 in FIG. 1A, from which it follows that the wave-matching recognition function described in connection with FIG. 1B is equally applicable here. This function is accomplished by rectifying (preferably full-wave) the signals 168 and 164' with rectifiers 214 and 216, respectively, and smoothing the rectified signals with respective R-C integrating circuits 218 and 220. The output voltages at the terminals 222 and 224 of the integrators are then subtracted from each other in a subtracting circuit 226, the difference shaped and limited in magnitude by a shaping network 228 (having the illustrated transfer function), again rectified with full-wave rectifier 230, and integrated with an integrator comprising a capacitor 232, a rise time control resistor 234, and a release or decay time control resistor 236. The signals 184 and 190 are similarly full-wave rectified in rectifiers 240 and 242, respectively, and integrated with respective R-C circuits 244 and 246, and the output voltages appearing at their respective output terminals 248 and 250 are subtracted from each other in subtracting circuit 252. The difference signal is shaped and limited by shaping network 254, having the same transfer function as shaping network 228, full-wave rectified by rectifier 256, and integrated in a circuit including resistors 258 and 260 and capacitor 262. The release (or discharge) time of integrators 218, 220, 244 and 246, as established by the product of their respective capacitances and resistances, preferably is of the order of 20 milliseconds, but this number is not critical.

The charge time of capacitors 232 and 262, established by resistors 236 and 260, respectively, and their decay times, established by resistors 234 and 258, respectively, are preferably of the order of 20 milliseconds and 250 milliseconds, respectively but these values, likewise, may vary over a considerable range without deteriorating system performance.

The outputs of the integrators, appearing at output terminals 264 and 266, respectively, are subtracted in an inversely complementary manner by a pair of subtracting circuits 268 and 270 which produce at their outputs control signals E, and E,,, respectively. The E, control signal is applied in parallel to the control terminals of amplifiers 178 and 180, which respectively control the signal applied to the front loudspeakers 170 and 176, and the E, control signal is applied in parallel to the control terminals of amplifiers 186 and 192, which control the amplitude of signals applied to the back loudspeakers 172 and 174, respectively. The gain control amplifiers, of a type similar to those described in connection with FIG. 1A, have a gain control characteristic similar to that illustrated in FIG. 2, although it should be understood that this is by way of example only. As in the system of FIG. 1, the gain control function is so established that the gain can rise relatively rapidly (e.g., in approximately 20 milliseconds) in response to a positively changing gain control voltage, but to diminish slowly (e.g., in approximately 800 milliseconds) when the gain control signal decreases. As with the other time control functions, these values may vary considerably without affecting the principle of operation, and, in essence, are adjusted to suit the artistic preference of the average listener.

It is desirable to maintain the range of the control voltages E, and E, within the limits indicated in FIG. 2, namely between -E for cut-off of amplification, and E,, for maximum gain, as there is no significant value in applying control voltages to the gain control amplifiers in excess of these limits. In the present embodiment, the amplitude of the control signals is achieved by summing the outputs appearing at the output terminals 222, 224, 248 and 250 in a summing circuit 272, and applying this sum signal in parallel to the gain control terminals 206 and 208 of automatic gain control amplifiers 202 and 204. In this manner, the gains of amplifiers 202 and 204 are automatically adjusted to accommodate for changes in amplitude of the input signals. For example, when the amplitude of signals L and R applied to input terminals 10 and 12 is low, the sum signal is in a direction to increase the gain of the amplifiers, whereas if the input level is high, the gain of the amplifiers is rapidly reduced so that the sum of the signals at terminals 222, 224, 248 and 250 is maintained at a relatively constant level.

The limits of the control signals E; and E, are further established by the wave-shaping networks 228 and 254, the transfer characteristics of which may be selected experimentally to obtain the most efficacious and pleasing action. A good guideline to this adjustment is that when a single signal L, and R applied to input terminals or 152, the gain of the front loudspeaker amplifiers 178 and 180 is increased to a maximum, and the gain of the back loudspeaker amplifiers 186 and 192 is just reduced to zero. For convenience in making the adjustment, it is preferable to provide a gain control adjusting knob for adjusting the transfer function of the wave-shaping networks 228 and 254. It is to be understood that the wave-shaping networks need not be placed as illustrated but, instead, may be placed in the output lines of subtractors 268 and 170.

I claim:

1. In combination:

decoder apparatus adapted to receive first and second composite signals L and R respectively containing dominant signals L; and R, and each including two sub-dominant signal components L, and R said signal component L, being of substantially equal magnitude and in substantially quadrature relationship in said first and second composite signals, and in one of said first and second composite signals in leading relationship with that in the other, and said signal component R, being of substantially equal magnitude and in substantially quadrature relationship in said first and second composite signals, and in said one of said first and second signals in lagging relationship with that in the other, and including at least two phase-shifting networks operative to shift the phase of one of said first and second composite signals relative to the other by substantially 9ll to cause the signal components L and R in one of said relatively phaseshifted composite signals to be in phase coincidence or in phase opposition with corresponding signal components in the other relatively phaseshifted composite signal, combining networks operative to derive from said relatively phase-shifted composite signals third and fourth composite sig nals respectively containing dominant signal components L and R and each including two subdom inant signal components L; and R said signal component L being of substantially equal magnitude and in phase coincidence or phase opposition in said third and fourth composite signals, and said signal component R; being of substantially equal magnitude and in phase coincidence or phase op position in said third and fourth composite signals, and means for applying composite signals respectively containing dominant signal components L,, R,, L and R to first, second, third and fourth gain control amplifiers, respectively, for reproduction over four corresponding loudspeaker circuits, and a control circuit for enhancing the realism of the four channel sound produced by the loudspeakers, siad control circuit comprising: first circuit means for comparing said relatively phase-shifted first and second composite signals and operative to produce a signal indicative of whether they contain substantially equal amplitude signals in phase coincidence or phase opposition,

second circuit means for comparing said third and fourth composite signals and operative to produce a signal indicative of whether they contain substantially equal amplitude signals in phase coincidence or phase opposition, and

third circuit means operative in response to signals from said first and second circuit means to enhance the gain of said first and second gain control amplifiers relative to said third and fourth gain control amplifiers when said relatively phase-shifted first and second composite signals do not contain said signal components L and R and to enhance the gain of said third and fourth gain control amplifiers relative to said first and second gain control amplifiers when said third and fourth signals do not contain said signal components L; and R 2. The combination defined by claim 1 wherein said first circuit means includes first and second rectifiers for separately rectifying said relatively phase-shifted first and second composite signals and means for subtracting one from the other the rectified outputs therefrom to produce a first difference signal, and said second circuit means includes third and fourth rectifiers for separately rectifying said third and fourth composite signals and means for subtracting one from the other the rectified outputs therefrom to produce a second difference signal.

3. The combination defined in claim 2 wherein said rectifiers are full-wave rectifiers and each includes means for integrating the output thereof.

4. The combination defined by claim 2 further including fifth and sixth rectifiers for separately rectifying said first and second difference signals, respectively.

5. The combination defined by claim 4 wherein said rectifiers are full-wave rectifiers and each includes means for integrating the output thereof.

lid

6. The combination defined by claim 2 wherein said third circuit means includes first and second subtracting means to both of which said first and second difference signals are applied in complementary subtracting relationship and operative to produce first and second control signals, respectively.

7. The combination defined by claim 6 wherein said control circuit further includes first and second waveshaping networks having substantially identical transfer functions for separately shaping said first and second difference signals.

8. The combination defined by claim 7 further including fifth and sixth rectifiers for separately rectifying said shaped first and second difference signals, respectively.

9. A system for processing sound reproducing signals comprising, in combination,

decoder means adapted to receive first and second composite signals L and R respectively containing dominant signals L, and R, and each including two sub-dominant signal components L,, and R said signal components L and R in said first composite signal being in substantially quadrature relationship with the corresponding signal components in said second composite signal, and including decoding means including at least first and second all-pass phase-shifting networks to which said first and second composite signals are respectively applied for shifting the phase of one of said first and second composite signals relative to the other by substantially and signal-combining networks for combining said relatively phase-shifted first and second composite signals to derive third and fourth composite signals respectively containing dominant signal components L and R means for applying composite signals respectively containing said signal components L], L,,, R and R, as predominant components to first, second, third and fourth gain control means, respectively, for reproduction over four corresponding loudspeaker circuits,

control circuit means operative in response to at least said relatively phase-shifted first and second composite signals to detect whether said relatively phase-shifted first and second composite signals contain substantially equal amplitude signal components in phase coincidence or in phase opposition and to produce a control signal operative to enhance the gain of the first and second gain control means when said relatively phase-shifted first and second composite signals do not contain said signal components L and R and means for applying said control signal to said first and second gain control means for controlling the gain thereof.

16. The combination in accordance with claim 9 wherein said third and fourth composite signals respectively contain dominant signal components L,, and R and two subdominant signal components L; and R,, said signal component R, being of substantially equal magnitude and in phase coincidence or phase opposition in said third and fourth composite signals, and said signal component R, being of substantially equal magnitude and in phase coincidence or phase opposition in said third and fourth composite signals, and said control circuit means further comprises first circuit means for comparing said first and second composite signals and operative to produce a signal indicative of whether they contain substantially equal amplitude signals in phase coincidence or phase opposition, and

second circuit means for comparing said third and fourth composite signals and operative to produce a signal indicative of whether they contain substantially equal amplitude signals in phase coincidence or phase opposition.

11. A system for receiving and processing sound reproducing signals comprising first, second, third and fourth gain control amplifiers connected to respectively receive first, second, third and fourth composite signals for reproduction over four corresponding loudspeaker circuits, the first, second, third and fourth composite signals containing respectively a dominant signal component a and subdominant signal components c and d, a dominant signal component b and subdominant signal components c and d, a dominant signal component 0 and sub-dominant signal components a and b, and a dominant signal component a and sub-dominant signal components a and b, and

a control circuit for the gain control amplifiers connected to receive said composite signals in a form such that in the first and second composite signals and also in the third and fourth composite signals like sub-dominant signal components are of substantially equal magnitude and are substantially in phase coincidence or phase oposition, the control circuit comprising:

a first circuit for comparing the first and second composite signals and operative to produce a signal indicative of whether they contain substantially equal amplitude signals which are either in phase coincidence or phase opposition,

a second circuit for comparing the third and fourth composite signals and operative to produce a signal indicative of whether they contain substantially equal amplitude signals which are either in phase coincidence or in phase opposition, and

a third circuit operative in response to signals from the first and second circuits to enhance the gain of the first and second gain control amplifiers relative to the third and fourth gain control amplifiers when the first and second composite signals do not contain the signal components 0 and d and to enhance the gain of the third and fourth gain control amplifiers relative to the first and second gain control amplifiers when the third and fourth composite signals do not contain the signal components a and b.

12. A system in accordance with claim 11, further including decoder apparatus for deriving the said first, second, third and fourth composite signals from first and second input signals respectively containing dominant signals a and b and each including two subdominant signal components 0 and d, the two signal components 0 being of substantially equal magnitude and in substantially quadrature phase relationship and the two signal components d being ofsubstantially equal magnitude and in substantially quadrature phase relationship, the signal components c and d in one of the input signals being in leading and lagging relationship, respectively with the signal components 0 and d in the other of the input signals,

said decoder apparatus including all-pass phaseshifting means operative to shift the phase of at least one of the input signals to obtain phaseshifted signals wherein the signal components 0 and d in one are in phase coincidence or in phase opposition with corresponding signal components in the other, and

signal combining networks operative to derive from the said phase-shifted signals the first, second, third and fourth composite signals for application to said gain control amplifiers.

13. in apparatus for reproducing on three 'or more sound-reproducing devices a like number of directional audio information signals contained in first and second composite signals each of which includes at least three of said audio information signals in preselected amplitude and phase relationships, two of which are common to both composite signals with said common signals in one of said composite signals in substantially quadra ture relationship with corresponding ones of said common signals in the other of said composite signals, the combination comprising:

decoding circuit means including at least first and second all-pass phase-shifting means connected to receive said first and second composite signals and operative in response thereto to shift the phase of one of said first and second composite signals relative to the other by substantially and to produce third and fourth composite signals each of which includes a combination of at least three of said audio information signals and respectively containing a different one of said common signals as its predominant component,

signal amplitude-modifying means connected to receive and operative to couple said first and second relatively phase-shifted first and second composite signals and said third and fourth composite signals to respective ones of said sound-reproducing devices,

control signal generating means connected to receive and operative in response to at least said relatively phase-shifted first and second composite signals to detect whether said relatively phase-shifted first and second composite signals contain substantially equal amplitude signal components in phase coincidence or in phase opposition and to produce a first control signal operative to enhance the gain of the signal amplitude-modifying means receiving said first and second relatively phase-shifted composite signals when said relatively phase-shifted first and second composite signals do not contain said common signals either in phase coincidence or in phase opposition, and

means for applying said first control signal to the signal amplitude-modifying means receiving said relatively phase-shifted first and second composite signals.

14. Apparatus according to claim 13 wherein said control signal generating means further comprises:

means connected to receive and operative to compare said third and fourth composite signals and to produce a second control signal when said third and fourth composite signals do not contain substantially equal signals either in phase coincidence or in phase opposition, and

means for applying said second control signal to the signal amplitude-modifying means receiving said third and fourth composite signals. 15. Apparatus according to claim 14 wherein said means for applying said first and second control signals to said signal amplitude-modifying means, respectively, includes means for subtracting said second control signal from said first control signal, and means for subtracting said first control signal from said second control signal.

16. Apparatus according to claim 13, wherein said control signal generating means includes means connected to receive said relatively phaseshifted first and second composite signals and operative to derive therefrom first, second, third and fourth auxiliary composite signals respectively containing in the same relative phase relationship the signal components contained in said relatively phaseshifted first and second composite signals and in said third and fourth composite signals,

first circuit means connected to receive and operative to compare said first and second auxiliary composite signals and to produce a first signal indicative of whether they contain substantially equal amplitude signals in phase coincidence or in phase opposition,

second circuit means connected to receive and operative to compare said third and fourth auxiliary composite signals and to produce a second signal indicative of whether they contain substantially equal amplitude signals in phase coincidence or in phase opposition,

means for subtracting said second signal from said first signal to produce said first control signal, and means for subtracting said first signal from said second signal to produce said second control signal.

17. Apparatus according to claim 16, wherein said first circuit means includes means for separately rectifying said first and second auxiliary composite signals and means for subtracting one from the other the rectified output signals therefrom to produce a first difference signal, and said second circuit means includes means for separately rectifying said third and fourth auxiliary composite signals and means for subtracting one from the other the rectified output signals therefrom to produce a second difference signal.

18. in a system for reproducing on four soundreproducing devices a like number of directional audio information signals contained in first and second composite signals each of which includes at least three of said audio information signals in preselected amplitude and phase relationships, two of which are common to both composite signals with said common signals in one of said composite signals in substantially quadrature relationship with corresponding ones of said common signals in the other of said composite signals, including decoding means connected to receive said first and second composite signals and operative to shift the phase of one of said composite signals relative to the other by substantially and to produce first, second, third and fourth output composite signals respectively containing a different one of said audio information signals as its predominant component and signal amplitudemodifying means connected to receive and operative to couple said first, second, third and fourth output composite signals to respective ones of said soundreproducing devices, a logic circuit for identifying the signal or signals instantaneously present in said output composite signals and for producing signals for controlling said signal amplitude-modifying means to enhance said instantaneously present signal or signals at said sound-reproducing devices relative to other signals, and logic circuit comprising:

means connected to receive and operative in response to at least said relatively phase-shifted first and second composite signals to detect whether said relatively phase-shifted first and second com posite signals contain substantially equal amplitude signal components in either phase coincidence or in phase opposition, and to produce a first control signal operative when applied to the signal amplitude-modifying means receiving said first and second output composite signals to enhance the gain thereof when said relatively phase-shifted first and second composite signals do not contain said common signals either in phase coincidence or in phase opposition. 19. A logic circuit according to claim 18 further including circuit means connected to receive and operative to compare said third and fourth output composite signals and to produce a second control signal operative when applied to the signal ampltiudemodifying means receiving said third and fourth output composite signals to enhance the gain thereof when said third and fourth output composite signals do not contain substantially equal signals either in phase coincidence or in phase opposition. 20. A logic circuit according to claim 18, wherein said last-mentioned means includes cicuit means operative in response to said relatively phase-shifted first and second composite signals to produce first, second, third and fourth auxiliary composite signals respectively containing in the same relative phase relationship the signal components contained in said first, second, third and fourth output composite signals, and means connected to receive and operative to compare said first and second auxiliary composite signals. 

1. In combination: decoder apparatus adapted to receive first and second composite signals LT and RT respectively containing dominant signals Lf and Rf and each including two sub-dominant signal components Lb and Rb, said signal component Lb being of substantially equal magnitude and in substantially quadrature relationship in said first and second composite signals, and in one of said first and second composite signals in leading relationship with that in the other, and said signal component Rb being of substantially equal magnitude and in substantially quadrature relationship in said first and second composite signals, and in said one of said first and second signals in lagging relationship with that in the other, and including at least two phase-shifting networks operative to shift the phase of one of said first and second composite signals relative to the other by substantially 90* to cause the signal components Lb and Rb in one of said relatively phase-shifted composite signals to be in phase coincidence or in phase opposition with corresponding signal components in the other relatively phase-shifted composite signal, combining networks operative to derive from said relatively phase-shifted composite signals third and fourth composite signals respectively containing dominant signal components Lb and Rb and each including two subdominant signal components Lf and Rf, said signal component Lf being of substantially equal magnitude and in phase coincidence or phase opposition in said third and fourth composite signals, and said signal component Rf being of substantially equal magnitude and in phase coincidence or phase opposition in said third and fourth composite signals, and means for applying composite signals respectively containing dominant signal components Lf, Rf, Lb and Rb to first, second, third and fourth gain control amplifiers, respectively, for reproduction over four corresponding loudspeaker circuits, and a control circuit for enhancing the realism of the four channel sound produced by the loudspeakers, siad control circuit comprising: first circuit means for comparing said relatively phase-shifted first and second composite signals and operative to produce a signal indicative of whether they contain substantially equal amplitude signals in phase coincidence or phase opposition, second circuit means for comparing said third and fourth composite signals and operative to produce a signal indicative of whether they contain substantially equal amplitude signals in phase coincidence or phase opposition, and third circuit means operative in response to signals from said first and second circuit means to enhance the gain of said first and second gain control amplifiers relative to said third and fourth gain control amplifiers when said relatively phaseshifted first and second composite signals do not contain said signal components Lb and Rb, and to enhance the gain of said third and fourth gain control amplifiers relative to said first and second gain control amplifiers when said third and fourth signals do not contain said signal components Lf and Rf.
 2. The combination defined by claim 1 wherein said first circuit means includes first and second rectifiers for separately rectifying said relatively phase-shifted first and second composite signals and means for subtracting one from the other the rectified outputs therefrom to produce a first difference signal, and said second circuit means includes third and fourth rectifiers for separately rectifying said third and fourth composite signals and means for subtracting one from the other the rectified outputs therefrom to produce a second difference signal.
 3. The combination defined in claim 2 wherein said rectifiers are full-wave rectifiers and each includes means for integrating the output thereof.
 4. The combination defined by claim 2 further including fifth and sixth rectifiers for separately rectifying said first and second difference signals, respectively.
 5. The combination defined by claim 4 wherein said rectifiers are full-wave rectifiers and each includes means for integrating the output thereof.
 6. The combination defined by claim 2 wherein said third circuit means includes first and second subtracting means to both of which said first and second difference signals are applied in complementary subtracting relationship and operative to produce first and second control signals, respectively.
 7. The combination defined by claim 6 wherein said control circuit further includes first and second wave-shaping networks having substantially identical transfer functions for separately shaping said first and second difference signals.
 8. The combination defined by claim 7 further including fifth and sixth rectifiers for separately rectifying said shaped first and second difference signals, respectively.
 9. A system for processing sound reproducing signals comprising, in combination, decoder means adapted to receive first and second composite signals LT and RT respectively containing dominant signals Lf and Rf and each including two sub-dominant signal components Lb and Rb, said signal components Lb and Rb in said first composite signal being in substantially quadrature relationship with the corresponding signal components in said second composite signal, and including decoding means including at least first and second all-pass phase-shifting networks to which said first and second composite signals are respectively applied for shifting the phase of one of said first and second composite signals relative to the other by substantially 90* and signal-combining networks for combining said relatively phase-shifted first and second composite signals to derive third and fourth composite signals respectively containing dominant signal components Lb and Rb, means for applying composite signals respectively containing said signal components Lf, Lb, Rb and Rf as predominant components to first, second, third and fourth gain control means, respectively, for reproduction over four corresponding loudspeaker circuits, control circuit means operative in response to at least said relatively phase-shifted first and second composite signals to detect whether said relatively phase-shifted first and sEcond composite signals contain substantially equal amplitude signal components in phase coincidence or in phase opposition and to produce a control signal operative to enhance the gain of the first and second gain control means when said relatively phase-shifted first and second composite signals do not contain said signal components Lb and Rb, and means for applying said control signal to said first and second gain control means for controlling the gain thereof.
 10. The combination in accordance with claim 9 wherein said third and fourth composite signals respectively contain dominant signal components Lb and Rb and two subdominant signal components Lf and Rf, said signal component Rf being of substantially equal magnitude and in phase coincidence or phase opposition in said third and fourth composite signals, and said signal component Rf being of substantially equal magnitude and in phase coincidence or phase opposition in said third and fourth composite signals, and said control circuit means further comprises first circuit means for comparing said first and second composite signals and operative to produce a signal indicative of whether they contain substantially equal amplitude signals in phase coincidence or phase opposition, and second circuit means for comparing said third and fourth composite signals and operative to produce a signal indicative of whether they contain substantially equal amplitude signals in phase coincidence or phase opposition.
 11. A system for receiving and processing sound reproducing signals comprising first, second, third and fourth gain control amplifiers connected to respectively receive first, second, third and fourth composite signals for reproduction over four corresponding loudspeaker circuits, the first, second, third and fourth composite signals containing respectively a dominant signal component a and subdominant signal components c and d, a dominant signal component b and sub-dominant signal components c and d, a dominant signal component c and sub-dominant signal components a and b, and a dominant signal component d and sub-dominant signal components a and b, and a control circuit for the gain control amplifiers connected to receive said composite signals in a form such that in the first and second composite signals and also in the third and fourth composite signals like sub-dominant signal components are of substantially equal magnitude and are substantially in phase coincidence or phase oposition, the control circuit comprising: a first circuit for comparing the first and second composite signals and operative to produce a signal indicative of whether they contain substantially equal amplitude signals which are either in phase coincidence or phase opposition, a second circuit for comparing the third and fourth composite signals and operative to produce a signal indicative of whether they contain substantially equal amplitude signals which are either in phase coincidence or in phase opposition, and a third circuit operative in response to signals from the first and second circuits to enhance the gain of the first and second gain control amplifiers relative to the third and fourth gain control amplifiers when the first and second composite signals do not contain the signal components c and d and to enhance the gain of the third and fourth gain control amplifiers relative to the first and second gain control amplifiers when the third and fourth composite signals do not contain the signal components a and b.
 12. A system in accordance with claim 11, further including decoder apparatus for deriving the said first, second, third and fourth composite signals from first and second input signals respectively containing dominant signals a and b and each including two subdominant signal components c and d, the two signal components c being of substantially equal magnitude and in substantially quadrature phase relationship and the two signal components d being of substantially equal magnitude and in substantially quadrature phase relationship, the signal components c and d in one of the input signals being in leading and lagging relationship, respectively with the signal components c and d in the other of the input signals, said decoder apparatus including all-pass phase-shifting means operative to shift the phase of at least one of the input signals to obtain phase-shifted signals wherein the signal components c and d in one are in phase coincidence or in phase opposition with corresponding signal components in the other, and signal combining networks operative to derive from the said phase-shifted signals the first, second, third and fourth composite signals for application to said gain control amplifiers.
 13. In apparatus for reproducing on three or more sound-reproducing devices a like number of directional audio information signals contained in first and second composite signals each of which includes at least three of said audio information signals in preselected amplitude and phase relationships, two of which are common to both composite signals with said common signals in one of said composite signals in substantially quadrature relationship with corresponding ones of said common signals in the other of said composite signals, the combination comprising: decoding circuit means including at least first and second all-pass phase-shifting means connected to receive said first and second composite signals and operative in response thereto to shift the phase of one of said first and second composite signals relative to the other by substantially 90* and to produce third and fourth composite signals each of which includes a combination of at least three of said audio information signals and respectively containing a different one of said common signals as its predominant component, signal amplitude-modifying means connected to receive and operative to couple said first and second relatively phase-shifted first and second composite signals and said third and fourth composite signals to respective ones of said sound-reproducing devices, control signal generating means connected to receive and operative in response to at least said relatively phase-shifted first and second composite signals to detect whether said relatively phase-shifted first and second composite signals contain substantially equal amplitude signal components in phase coincidence or in phase opposition and to produce a first control signal operative to enhance the gain of the signal amplitude-modifying means receiving said first and second relatively phase-shifted composite signals when said relatively phase-shifted first and second composite signals do not contain said common signals either in phase coincidence or in phase opposition, and means for applying said first control signal to the signal amplitude-modifying means receiving said relatively phase-shifted first and second composite signals.
 14. Apparatus according to claim 13 wherein said control signal generating means further comprises: means connected to receive and operative to compare said third and fourth composite signals and to produce a second control signal when said third and fourth composite signals do not contain substantially equal signals either in phase coincidence or in phase opposition, and means for applying said second control signal to the signal amplitude-modifying means receiving said third and fourth composite signals.
 15. Apparatus according to claim 14 wherein said means for applying said first and second control signals to said signal amplitude-modifying means, respectively, includes means for subtracting said second control signal from said first control signal, and means for subtracting said first control signal from said second control signal.
 16. Apparatus according to claim 13, wherein said control signal generating means includes means connected to receive said relatively phase-shifted first and second composite signals and operative to derive therefrom first, second, third and fourth auxiliary composite signals respectively containing in the same relative phase relationship the signal components contained in said relatively phase-shifted first and second composite signals and in said third and fourth composite signals, first circuit means connected to receive and operative to compare said first and second auxiliary composite signals and to produce a first signal indicative of whether they contain substantially equal amplitude signals in phase coincidence or in phase opposition, second circuit means connected to receive and operative to compare said third and fourth auxiliary composite signals and to produce a second signal indicative of whether they contain substantially equal amplitude signals in phase coincidence or in phase opposition, means for subtracting said second signal from said first signal to produce said first control signal, and means for subtracting said first signal from said second signal to produce said second control signal.
 17. Apparatus according to claim 16, wherein said first circuit means includes means for separately rectifying said first and second auxiliary composite signals and means for subtracting one from the other the rectified output signals therefrom to produce a first difference signal, and said second circuit means includes means for separately rectifying said third and fourth auxiliary composite signals and means for subtracting one from the other the rectified output signals therefrom to produce a second difference signal.
 18. In a system for reproducing on four sound-reproducing devices a like number of directional audio information signals contained in first and second composite signals each of which includes at least three of said audio information signals in preselected amplitude and phase relationships, two of which are common to both composite signals with said common signals in one of said composite signals in substantially quadrature relationship with corresponding ones of said common signals in the other of said composite signals, including decoding means connected to receive said first and second composite signals and operative to shift the phase of one of said composite signals relative to the other by substantially 90* and to produce first, second, third and fourth output composite signals respectively containing a different one of said audio information signals as its predominant component and signal amplitude-modifying means connected to receive and operative to couple said first, second, third and fourth output composite signals to respective ones of said sound-reproducing devices, a logic circuit for identifying the signal or signals instantaneously present in said output composite signals and for producing signals for controlling said signal amplitude-modifying means to enhance said instantaneously present signal or signals at said sound-reproducing devices relative to other signals, and logic circuit comprising: means connected to receive and operative in response to at least said relatively phase-shifted first and second composite signals to detect whether said relatively phase-shifted first and second composite signals contain substantially equal amplitude signal components in either phase coincidence or in phase opposition, and to produce a first control signal operative when applied to the signal amplitude-modifying means receiving said first and second output composite signals to enhance the gain thereof when said relatively phase-shifted first and second composite signals do not contain said common signals either in phase coincidence or in phase opposition.
 19. A logic circuit according to claim 18 further including circuit means connected to receive and operative to cOmpare said third and fourth output composite signals and to produce a second control signal operative when applied to the signal ampltiude-modifying means receiving said third and fourth output composite signals to enhance the gain thereof when said third and fourth output composite signals do not contain substantially equal signals either in phase coincidence or in phase opposition.
 20. A logic circuit according to claim 18, wherein said last-mentioned means includes cicuit means operative in response to said relatively phase-shifted first and second composite signals to produce first, second, third and fourth auxiliary composite signals respectively containing in the same relative phase relationship the signal components contained in said first, second, third and fourth output composite signals, and means connected to receive and operative to compare said first and second auxiliary composite signals. 